Don't allow combination of -g and --reverse as it doesn't work

The --walk-reflogs logic and the --reverse logic are completely
incompatible with one another.  Attempting to use both at the same
time leads to confusing results that sometimes violates the user's
formatting options or ignores the user's request to see the reflog
message and timestamp.

Unfortunately the implementation of both of these features is glued
onto the side of the revision walking machinary in such a way that
they are probably not going to be easy to make them compatible with
each other.  Rather than offering the user confusing results we are
better off bailing out with an error message until such a time as
the implementations can be refactored to be compatible.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-log.txt

@@ -49,6 +49,7 @@ include::pretty-options.txt[]
 -g, \--walk-reflogs::
 	Show commits as they were recorded in the reflog. The log contains
 	a record about how the tip of a reference was changed.
+	Cannot be combined with --reverse.
 	See also gitlink:git-reflog[1].
 
 --decorate::

Documentation/git-rev-list.txt

@@ -298,6 +298,8 @@ used in the output.  When the starting commit is specified as
 'commit@{now}', output also uses 'commit@\{timestamp}' notation
 instead.  Under '\--pretty=oneline', the commit message is
 prefixed with this information on the same line.
++
+Cannot be combined with --reverse.
 
 --merge::
 
@@ -373,6 +375,7 @@ By default, the commits are shown in reverse chronological order.
 --reverse::
 
 	Output the commits in reverse order.
+	Cannot be combined with --walk-reflogs.
 
 Object Traversal
 ~~~~~~~~~~~~~~~~

revision.c

@@ -1277,6 +1277,9 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, const ch
 		compile_grep_patterns(revs->grep_filter);
 	}
 
+	if (revs->reverse && revs->reflog_info)
+		die("cannot combine --reverse with --walk-reflogs");
+
 	return left;
 }